This invention is related to the subject matter of a U.S. patent application entitled PROCESS FOR SHARING AN UPSTREAM AMONG MULTIPLE DOWNSTREAMS, Ser. No. 10/295,712, filed Nov. 15, 2002 (TER-029) and a U.S. patent application entitled CABLE MODEM TERMINATION SYSTEM WITH FLEXIBLE ADDITION OF SINGLE UPSTREAMS OR DOWNSTREAMS, filed Nov. 29, 2003, Ser. No. 10/725,092 (TER-045) both of which are hereby incorporated by reference.
In today's cable modem termination systems (CMTS), there is a fixed ratio of downstreams to upstreams on each expansion card. This means that a prior art expansion card for a CMTS has one downstream transmitter with all the media access control and physical layer circuitry needed to transmit a DOCSIS downstream on a hybrid fiber coaxial cable (HFC). The prior art expansion cards also has four upstream receivers, each with the MAC and physical layer circuitry needed to interface with one hybrid fiber coaxial cable medium. Other prior art expansion cards have different ratios of upstreams to downstreams. However, the need for additional upstream and downstream capacity does not change in the same fixed ratio as the cards. Thus, when upstream or downstream installed capacity is exceeded, another card must be added which adds another downstream and four upstreams. This causes excessive expenditure and bad return on investment if all the cable operator needs is another upstream or another downstream. A need has arisen for a system which can smoothly and gradually add downstream or upstream capacity as needed. As downstreams are added, it would be convenient to be able to associate them to a shared upstream. A U.S. patent application entitled PROCESS FOR SHARING AN UPSTREAM AMONG MULTIPLE DOWNSTREAMS, Ser. No. 10/295,712, filed Nov. 15, 2002 teaches how to do this and teaches a CMTS mapping system at FIG. 11. This CMTS mapping system maps M downstreams flexibly to N HFC systems. In other words, it has the ability to split a single downstream for simultaneous transmission thereof on more than one HFC and to combine multiple downstreams for transmission on a single HFC (or multiple HFC systems that are different in number than the number of downstreams which have been combined). The CMTS mapping system also has the ability to allow multiple downstreams to share a single upstream receiver, and the ability to combine an upstream arriving on multiple HFC systems for coupling to an input of a single upstream receiver.
When sharing an upstream receiver, one option is to combine upstream logical channels and apply the combined signals to the input of a shared receiver. This aggregates the noise on the upstreams. When there are multiple receivers, summing all the inputs to one receiver is a waste. Further aggregation of noise is not necessary during either contention intervals or non contention intervals. A need has arisen for a method and apparatus to share receivers without aggregation of noise during contention and non contention intervals and a way to fully utilized all receivers during contention intervals and reduce noise aggregation.
Further, it would be desirable to share a downstream between multiple upstreams and to do load balancing as traffic conditions dictate. This notion of flexible addition of upstreams and downstream, removing the necessity for a line card to have both upstreams and downstream or fixed ratios there between, and flexible mapping between upstreams and downstreams will be referred to as upstream/downstream decoupling. Decoupling is somewhat misdescriptive because the downstreams which share an upstream must be coordinated tightly as will be described below. The term is meant here to refer to the elimination of the need to add unneeded circuitry as the need for capacity grows by providing a greater granularity in the ability to add single downstream transmitters or single upstream receivers, map multiple downstreams to the same upstream or multiple upstreams to the same downstream, and coordinate the media access control messaging needed to do that.
Further, it would be desirable to provide a CMTS which supports the coexistence of MPEG video delivery via conventional transport streams with video delivery over IP packets, and provide two way conditional access and high definition delivery capability and video rate shaping.